6.8 The yield statement

The yield statement is only used when defining a generator
function, and is only used in the body of the generator function.
Using a yield statement in a function definition is
sufficient to cause that definition to create a generator function
instead of a normal function.

When a generator function is called, it returns an iterator known as a
generator iterator, or more commonly, a generator. The body of the
generator function is executed by calling the generator's
next() method repeatedly until it raises an exception.

When a yield statement is executed, the state of the
generator is frozen and the value of expression_list is
returned to next()'s caller. By ``frozen'' we mean that all
local state is retained, including the current bindings of local
variables, the instruction pointer, and the internal evaluation stack:
enough information is saved so that the next time next() is
invoked, the function can proceed exactly as if the yield
statement were just another external call.

As of Python version 2.5, the yield statement is now
allowed in the try clause of a try ...
finally construct. If the generator is not resumed before
it is finalized (by reaching a zero reference count or by being garbage
collected), the generator-iterator's close() method will be
called, allowing any pending finally clauses to execute.

Note:
In Python 2.2, the yield statement is only allowed
when the generators feature has been enabled. It will always
be enabled in Python 2.3. This __future__ import statement can
be used to enable the feature: